The present invention relates to an improved heat exchanger using bayonet tubes and more particularly an improved heat exchanger free of thermal stress, comprising bayonet tube outer ducts which are open at and secured to a tube sheet of the heat exchanger and bayonet tube inner ducts which are open to and secured to a high temperature fluid separation chamber of the heat exchanger.
In chemical plants, heat exchangers are used for the recovery of heat from high temperature gas generated as a result of burning, a reaction or the like.
Normal heat exchangers conventionally used are such as those shown in FIG. 1, and comprise a shell 1 containing a plurality of tubes 2 therein, the ends of the shell 1 being enclosed by tube sheets 3, 3 with the tubes 2 passing through the tube sheets and opened to chambers which are enclosed by the stationary heads 4,4 and the tube sheets 3,3. The shell 1 is provided with an inlet nozzle 5 and an outlet nozzle 6 for the first fluid. The stationary head 4 on one side of the shell is provided with an inlet nozzle 7 for the second fluid, and the stationary head 4 on the other side is provided with an outlet nozzle 8 for the second fluid. When heat exchangers of this type are used, the shell 1 is in contact with the first fluid, while the tubes 2 are in contact with the second fluid. Therefore the temperature difference therebetween causes a change in relative thermal expansion between the shell 1 and the tubes 2. Thermal stress is thereby induced at the connection between the tubes 2 and the tube sheets 3 and at the connection between the shell 1 and the tube sheets 3. The temperature difference also exists between the inner and outer surfaces of the tube sheets 3. The thermal stress caused by those temperature conditions often makes the design of heat exchangers of this type difficult. Further, the places where thermal stress arises as mentioned above are located where inspection as well as repair is difficult to perform.
In order to absorb the thermal expansion it is possible to provide the middle portion of the shell with an expansion joint 9. However, if the first fluid is a hot gas, insulation materials which are lined on the shell wall would separate therefrom due to the expansion and contraction of the shell 1. And if the aforementioned first fluid is water, high pressure steam exceeding 100 atoms is normally generated, thereby rendering the mechanical design of expansion joints very difficult.
Another type of conventionally used heat exchangers is shown in FIG. 2. It comprises a shell 1 having an inlet nozzle 5 and an outlet nozzle 6 for the first fluid wherein U tubes 2a are contained, the ends of the U tubes 2a passing through a tube sheet 3 and being open to a chamber defined by a tube sheet 3, a stationary head 4a and a chamber cover 4b. The chamber is separated into two volumes by a pass partition 10, one volume being provided with an inlet nozzle 7 for the second fluid and an open port of one end of each of the U tubes 2a, the other volume being provided with an outlet nozzle 8 for the second fluid and an open port of the other end of each of the U tubes 2a. In this case, there is no problem of thermal expansion which is caused by the temperature difference between the shell 1 and the U tubes 2a, but since the chamber is divided into two volumes by the pass partition 10, the hot second fluid flows into one volume, and the cold second fluid after exchanging heat flows into the other volume, the big temperature difference prevailing along the tube sheet 3, causing thermal stress to arise therein, which makes the selection of structural materials and establishment of safe design very difficult.